Nickel-metal hydride storage battery and production method thereof

ABSTRACT

In order to obtain a nickel-metal hydride storage battery having a long cycle life and a low self-discharge rate, in a nickel-metal hydride storage battery comprising: a positive electrode containing nickel hydroxide; a negative electrode containing a hydrogen storage alloy; a separator interposed between the positive and negative electrodes; and an electrolyte comprising an aqueous alkaline solution, a water absorbent polymer, a water repellent and an aqueous alkaline solution are added into the separator. This battery can be produced with low cost.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Section 371 of International Application No.PCT/JP01/05733, filed Jul. 2, 2001, the disclosure of which isincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a nickel-metal hydride storage battery.More specifically, the present invention relates to a nickel metalhydride storage battery having an improved separator.

BACKGROUND ART

Conventional nickel-metal hydride storage batteries are composed of apositive electrode containing nickel hydroxide, a negative electrodecontaining a hydrogen storage alloy, a separator interposed between thepositive and negative electrodes, and an electrolyte. In the separator,a non-woven fabric made of polyolefin is used, and in the electrolyte,an aqueous solution of potassium hydroxide is used (Power Sources 12,Research and Development in Non-mechanical Electrical Power Sources,1989, p 393–410.).

The conventional nickel-metal hydride storage batteries have a problemsuch that, during the repeated charge/discharge cycles at a hightemperature, the hydrogen storage alloy contained in the negativeelectrode is corroded to produce an oxide or a hydroxide because thealloy reacts with the electrolyte. This reaction consumes the water inthe electrolyte. Accordingly, when the alloy is corroded, the amount ofthe electrolyte within the separator is decreased to increase theinternal resistance of the battery, resulting in lower cyclecharacteristic. Additionally, since the constituent element of thehydrogen storage alloy dissolved in the electrolyte can migrate to thepositive electrode through the separator, the self-discharge of thebattery is accelerated.

In order to cope with the corrosion of the hydrogen storage alloy,changes in the hydrogen storage alloy composition, surface treatments ofthe alloy and the like have been examined.

In order to deal with the increase in the internal resistance of abattery, improvements in the separator has been investigated. If thehydrophilicity of the separator is improved, the internal resistance ofthe battery is unlikely to increase even if the amount of theelectrolyte in the battery is decreased. This, however, arises problemssuch as the requirement of a step of enhancing the hydrophilicity of theseparator, and the increase in the production cost of the battery.

With regard to the self-discharge, Japanese Laid-Open Patent PublicationNo. Hei 5-258767 proposes to include a water absorbent polymer in anelectrolyte in order to reduce self-discharge during storage of thebattery. This method, however, has a disadvantage that the waterabsorbent polymer is distributed unevenly between the positive andnegative electrodes. The uneven distribution of the water absorbentpolymer leads to uneven proceeding in the battery reaction; thus, thecycle life of the battery cannot be expected to improve.

U.S. Pat. No. 5,541,019 proposes to use a polymer electrolyte in anickel-metal hydride storage battery in order to prevent the leakage ofthe electrolyte. The polymer electrolyte has low gas permeability.Accordingly, a sealed nickel-metal hydride storage battery containing apolymer electrolyte has a disadvantage that the internal pressure of thebattery tend to increase if a gas is generated by decomposition of waterdue to overcharge.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a low-cost nickel-metalhydride storage battery with a longer cycle life and lessself-discharge.

The present invention relates to a nickel-metal hydride storage batterycomprising a positive electrode containing nickel hydroxide, a negativeelectrode containing a hydrogen storage alloy, and a separatorcontaining a gel electrolyte interposed between the positive andnegative electrodes, characterized in that the gel electrolyte comprisesa water absorbent polymer, a water repellent and an aqueous alkalinesolution.

The water absorbent polymer comprises a cross-linked polymer having atleast one kind of monomer unit selected from the group consisting of anacrylic acid salt unit and a methacrylic acid salt unit.

The water repellent comprises at least one selected from the groupconsisting of fluorinated carbon and fluorocarbon resin. As thefluorocarbon resin, for instance, polytetrafluoroethylene can be used.

The separator may comprise only the gel electrolyte. The separator mayalso contain a core material comprising a non-woven fabric. Thenon-woven fabric preferably comprises polyolefin or polyamide.

The separator is preferably attached to a surface of at least one of thepositive and negative electrodes.

It is preferred that the separator further contains a binder in order toimprove the moldability and the durability thereof. The binder comprisesat least one selected from the group consisting of polyethylene,polypropylene, carboxymethyl cellulose, styrene butadiene rubber andpolyvinyl alcohol.

The separator layer preferably has an air permeability of 5 to 100ml/cm²·s under a pressure difference of 120 to 130 Pa.

The separator layer preferably has a thickness of 20 to 200 μm.

The present invention further relates to a method for producing anickel-metal hydride storage battery comprising: (1) a first step ofproducing a positive electrode containing nickel hydroxide and anegative electrode containing a hydrogen storage alloy; (2) a secondstep of producing a separator containing a gel electrolyte from amixture obtained by mixing a water absorbent polymer, a water repellentand an aqueous alkaline solution; (3) a third step of obtaining anelectrode group by laminating the positive and negative electrodes withthe separator interposed therebetween; and (4) a forth step ofassembling a nickel-metal hydride storage battery by using the electrodegroup.

The second step may comprise a step of forming a sheet-like separatorfrom a mixture obtained by mixing a water absorbent polymer, a waterrepellent and an aqueous alkaline solution.

The second step may also comprise a step of applying a mixture obtainedby mixing a water absorbent polymer, a water repellent and an aqueousalkaline solution on a surface of at least one of the positive andnegative electrodes, and thereby forming a separator attached to thesurface.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a longitudinal sectional view of one example of a nickel-metalhydride storage battery of the present invention.

FIG. 2 is a graph showing a relationship between the discharge capacityand the charge/discharge cycle number of Batteries A and B.

BEST MODE FOR CARRYING OUT THE INVENTION

A nickel-metal hydride storage battery of the present inventioncomprises a positive electrode containing nickel hydroxide, a negativeelectrode containing a hydrogen storage alloy, and a separatorcontaining a gel electrolyte comprising a water absorbent polymer, awater repellent and an aqueous alkaline solution interposed between thepositive and negative electrodes.

The separator has proper ion conductivity because it contains an aqueousalkaline solution.

The separator contains a gel electrolyte comprising a water absorbentpolymer and an aqueous alkaline solution. Therefore, the separator hasan excellent retention of an aqueous alkaline solution, and thebattery's internal resistance is unlikely to rise. In addition, sincethe electrolyte is in a gel form, the corrosion of the hydrogen storagealloy is unlikely to proceed; thus, the battery's self-discharge issuppressed.

As the water absorbent polymer, any polymer having a hydrophilic groupcan be used without specific limitation. For example, an alkali metalsalt of a polymer can be used. Examples of the polymer includepolyacrylic acid, polymethacrylic acid, a copolymer of acrylic acid andmethacrylic acid, a copolymer of isobutylene and maleic acid,poly(2-acrylamide-2-metylpropanesulfonic acid), poly(acryloxypropanesulfonic acid), poly(vinyl phosphonic acid). These polymers have a lotof acidic groups, but all acidic groups do not necessarily need to formthe alkali metal salt. The water absorbent polymers may be used singlyor in combination of two or more. As the water absorbent polymer,potassium polyacrylate, sodium polyacrylate, potassium polymethacrylateand sodium polymethacrylate are particularly preferred.

The water absorbent polymer is preferably a cross-linked polymer. Inorder to cross-link a water absorbent polymer, for instance, across-linking agent such as divinyl-benzene may be added during thepreparation of a polymer such as polyacrylic acid, polymethacrylic acidand a copolymer of acrylic acid and methacrylic acid. Alternatively, anionomer obtained by crosslinking a polymer such as polyacrylic acidspolymethacrylic acid, and a copolymer of acrylic acid and methacrylicacid with metal ions may be used.

The separator has excellent gas permeability because it contains a waterrepellent. Accordingly, even if the battery is overcharged, the internalpressure is unlikely to be high.

Examples of the water repellent are fluorinated carbon and fluorocarbonresin.

The molar ratio of fluorine atoms to carbon atoms contained in thefluorinated carbon is usually 1:1. A fluorinated carbon represented byCF_(x)(x<1) may also be used.

As the fluorocarbon resin, for instance, polytetrafluoroethylene can beused.

The separator can comprise only a gel electrolyte, but it may contain aconventional core material. As the core material, a non-woven fabriccomprising polyolefin or polyamide can be used. With the use of suchcore material, the tensile strength of the separator is increased,resulting in easy handling of the separator. Accordingly, the separatorwith a core material is unlikely to be damaged even when it is woundwith positive and negative electrodes sandwiching the separator.

The separator with a core material can be obtained by impregnating thecore material with a water absorbent polymer, or applying a waterabsorbent polymer onto the core material.

Usually, a non-woven fabric cannot be used as the separator unless it issubjected to a hydrophilic treatment or the like to enhance liquidabsorptivity. When a separator obtained by impregnating a non-wovenfabric with a water absorbent polymer, or applying a water absorbentpolymer onto a non-woven fabric is used, however, it is unnecessary tosubject the non-woven fabric to a hydrophilic treatment because thewater absorbent polymer is hydrophilic. Accordingly, in the presentinvention, the production cost of the battery can be reduced.

A separator preferably has a thickness of 5 to 200 μm. If the thicknessis too thin, the strength of the separator becomes insufficient, causinga problem such as an internal short circuit in the battery. On thecontrary, if the separator has a thickness of over 200 μm, otherproblems arise such as a bulky battery, small air permeability of theseparator and large internal resistance of the battery.

Next, one example of the method for producing a nickel-metal hydridestorage battery of the present invention will be described referring toFIG. 1. FIG. 1 is a longitudinal sectional view of one example of thecylindrical nickel-metal hydride storage battery of the presentinvention. In FIG. 1, the numerals 1, 2 and 3 respectively represents apositive electrode containing nickel hydroxide, a negative electrodecontaining a hydrogen storage alloy and a separator.

The positive electrode 1 can be obtained by applying a positiveelectrode material mixture onto a current collector. The negativeelectrode 2 can be obtained by applying a negative electrode materialmixture onto a current collector. As the current collector, a metalfoil, expanded metal or the like can be used.

The positive electrode 1 and the negative electrode 2 can be produced bya conventional method. The produced positive and negative electrodes areusually in the shape of a belt.

The separator 3 contains a gel electrolyte comprising a water absorbentpolymer, a water repellent and an aqueous alkaline solution. Asdescribed above, the separator 3 may have a core material made of anon-woven fabric.

In order to form the separator, first, a water absorbent polymer, awater repellent and an aqueous alkaline solution are mixed. A binder maybe added to the mixture in this stage if necessary.

The amount of the water repellent per 100 parts by weight of the totalamount of the water absorbent polymer and the aqueous alkaline solutionis preferably 1 to 8 parts by weight. The amount of the binder per 100parts by weight of the total amount of the water absorbent polymer andthe aqueous alkaline solution is preferably 0.1 to 2 parts by weight.When the amount of the water repellent is too small, the gaspermeability of the separator will be small. On the contrary, when theamount of the water repellent is too large, the internal resistance ofthe battery will be large. The amount of the aqueous alkaline solutionmay be changed according to the kind of water absorbent polymer. Theaqueous alkaline solution preferably has a specific gravity of 1.1 to1.4 g/ml.

The mixture comprising the water absorbent polymer, the water repellentand the aqueous alkaline solution is uniformly applied onto a surface ofa substrate whose surface is smooth, which is then dried to some extentto give a gel. A desired separator can be obtained by peeling theobtained gel away from the substrate. Alternatively, the above-mentionedmixture is impregnated into a non-woven fabric, or applied onto anon-woven fabric, which is then dried to some extent to give a separatorhaving the non-woven fabric as a core material.

Then, the obtained positive electrode 1 and negative electrode 2 arelaminated with the separator 3 interposed therebetween, which is thenspirally wound to form an electrode group. The electrode group is housedin a battery case 5 after an insulating plate 4 is provided at thebottom of the electrode group. Subsequently, an aqueous alkalinesolution is fed in the battery case 5.

The mixture comprising the water absorbent polymer, the water repellentand the aqueous alkaline solution may be applied directly on one surfaceor both surfaces of the electrode plate. In this case, the formedseparator is closely attached to the electrode surface. When theelectrode plate with the separator closely attached is used, it isunnecessary to laminate three members including the positive andnegative electrodes and the separator, and wind them. In addition, theseparator is unlikely to slide.

The opening of the battery case 5 is sealed with a sealing member 8. Thesealing member 8 is integrated with a cap 6 having a positive electrodeterminal. The sealing member 8 is further equipped with a safety valvemade of a rubber 7 which blocks a hole communicating the inside and theoutside of the battery case. The sealing member 8 also has an insulatinggasket 9 on the periphery thereof. The gasket 9 is provided in order toseal the battery as well as to insulate the positive electrode terminalfrom the negative electrode terminal. If a gas generates in the batteryand the internal pressure increases, the rubber 7 deforms, thereby thegas is released through the hole communicating the inside and theoutside of the battery case.

The positive electrode constituting the electrode group has a positiveelectrode lead 10. The positive electrode lead 10 is connected to thepositive electrode terminal of the sealing member 8. Part of thenegative electrode located in the outermost of the electrode groupcontacts the inner face of the battery case 5 made of metal. The outersurface of the battery case is covered with an insulating materialexcept the bottom. The bottom outer surface of the battery case servesas the negative electrode terminal.

In the following, the present invention will be concretely describedbased on Examples.

EXAMPLE 1

(i) Production of Positive Electrode

A nickel hydroxide containing Co and Zn was used as the positiveelectrode active material. A mixture obtained by mixing 100 parts byweight of the active material, 10 parts by weight of cobalt hydroxideand an appropriate amount of water was filled into micropores of foamednickel sheet with a thickness of 1.2 mm. The foamed nickel sheet inwhich the mixture was filled was dried, rolled out and cut to give apositive electrode. A positive electrode lead was provided to thepositive electrode.

(ii) Production of Negative Electrode

As the negative electrode material, a well-known AB₅ type hydrogenstorage alloy was used. The hydrogen storage alloy was ground intoparticles with a mean particle size of 35 μm, which was then treatedwith alkali. Subsequently, the treated alloy powder was mixed with anappropriate amount of binder and water. The obtained mixture was appliedto a punched metal substrate plated with nickel. The substrate appliedwith the mixture was dried, rolled out and cut to give a negativeelectrode. A negative electrode lead was provided to the negativeelectrode.

(iii) Production of Separator

10 g of cross-linked potassium polyacrylate, 125 g of aqueous solutionof potassium hydroxide with a specific gravity of 1.25 g/ml, 0.1 g ofcarboxymethyl cellulose and 6.75 g of polytetrafluoroethylene powderwere mixed and then gelated. The obtained gel was applied on a surfaceof a glass plate whose surface is smooth, which was then dried and theresulting sheet was peeled away. The obtained sheet of gel was rolledout until it reached a thickness of 150 μm, and cut to give a separator.The obtained separator had an air permeability of 20 ml/cm²·s under apressure difference of 124 Pa.

(iv) Assembly of Battery

The positive and negative electrodes were laminated with the separatorinterposed therebetween, which was then wound to give an electrodegroup. The electrode group was provided with a ring-like insulatingplate at the bottom thereof, which was then housed in an AA sizedbattery case. The negative electrode lead was spot-welded to the bottomof the battery case. Then, an aqueous solution of potassium hydroxidewith a specific gravity of 1.3 g/ml was fed into the battery case as anelectrolyte. An insulating plate was placed on the top of the electrodegroup. Finally, the opening of the battery case was sealed with asealing member equipped with a cap having a positive electrode terminal,a safety valve and a gasket. Before the sealing, the positive electrodelead was electrically connected to the positive electrode cap. Thereby,a sealed battery was produced. This battery was referred to as “BatteryA”.

(v) Battery Evaluation

Six batteries A with an initial nominal capacity of 1200 mAh wereprepared. Three of them were put through the repetition of charge anddischarge at 45° C. in order to determine the cycle life. The chargecurrent was 0.1 A, the charge time was 12 hours, the interval betweencharge and discharge was 1 hour, the discharge current was 0.2 A, andthe end-of-discharge voltage was 0.8 V. As a result, the dischargecapacity at the initial charge/discharge cycle was about 1000 mAh. Inaddition, the average cycle number needed to reach a discharge capacityof 500 mAh was 350.

Next, the other three batteries were used to determine theself-discharge characteristic. First, at a temperature of 20° C., eachbattery was charged at 120 mA for 15 hours and discharged at 240 mA to abattery voltage of 1.0 V; thereafter, the discharge capacity of eachbattery was measured. Subsequently, the each battery was charged at 120mA for 15 hours at 20° C., which was then stored for 30 days at 45° C.When the battery temperature dropped to 20° C. after the storage, thedischarge capacity was measured in the same manner as above. Eventually,the percentage (%) of the battery capacity obtained after the storage at45° C. for 30 days to that before the storage was calculated. Theobtained result was referred to as “self-discharge rate”. The averageself-discharge rate of Battery A was 20%.

Meanwhile, a separator having an air permeability of less than 5ml/cm²·s was produced, which was then used to manufacture a battery suchas that in Example 1. The obtained battery was evaluated in the samemanner as Example 1 to find that the self-discharge rate was decreased.When the charge/discharge cycle was repeated, however, the internalpressure of the battery was increased; thus, the cycle life was notincreased.

Similarly, a separator having an air permeability of greater than 100ml/cm²·s was produced, which was then used to manufacture a battery suchas that in Example 1. The obtained battery was evaluated in the samemanner as Example 1 to find that the self-discharge rate was decreased.However, the initial discharge capacity was decreased to some extent.

Comparative Example 1

A battery was produced in the same manner as Example 1, except that ahydrophilic-treated polypropylene non-woven fabric was used, instead ofusing the separator obtained in Example 1. This battery was denoted as“Battery B”.

Battery B was evaluated in the same manner as Example 1. The dischargecapacity of Battery B at the initial charge/discharge cycle was about1000 mAh.

The average cycle number needed to reach a discharge capacity of 500 mAhwas 250, which was about 100 cycles lower than that of Battery A.

The average self-discharge rate of Battery B was 38%, which was 18%higher than that of Battery A.

The above results show that Battery A extremely excels in cycle life andself-discharge characteristics as compared to Battery B. FIG. 2 shows arelationship between the discharge capacity and the charge/dischargecycle number of Battery A and B.

EXAMPLE 2

A battery was produced in the same manner as Example 1, except thatfluorinated carbon (CF_(1.0)) was used instead ofpolytetrafluoroethylene when producing the separator. This battery wasdenoted as “Battery C”.

Battery C was evaluated in the same manner as Example 1. The dischargecapacity of Battery C at the initial charge/discharge cycle was about1000 mAh.

The average cycle number needed to reach a discharge capacity of 500 mAhwas 355.

EXAMPLE 3

A mixture was prepared by mixing 10 g of crosslinked potassiumpolyacrylate, 125 g of aqueous solution of potassium hydroxide with aspecific gravity of 1.25 g/ml, 0.1 g of carboxymethyl cellulose and 6.75g of polytetrafluoroethylene powder. The obtained mixture was applied onthe polypropylene non-woven fabric used in Comparative Example 1, whichwas then dried to give a separator with a thickness of 150 μm. Theobtained separator had an air permeabililty of about 15 ml/cm²·s under apressure difference of 124 Pa.

A battery was produced in the same manner as Example 1, except that thisseparator was used. This battery was denoted as “Battery D”.

Battery D was evaluated in the same manner as Example 1.

The discharge capacity of Battery D at the initial charge/dischargecycle was about 1000 mAh.

The average cycle number needed to reach a discharge capacity of 500 mAhwas 320.

EXAMPLE 4

A mixture was prepared by mixing 10 g of crosslinked potassiumpolyacrylate, 125 g of aqueous solution of potassium hydroxide with aspecific gravity of 1.25 g/ml, 0.1 g of carboxymethyl cellulose and 6.75g of polytetrafluoroethylene powder. The obtained mixture was applied onthe both surfaces of the positive and negative electrodes used inExample 1, which was dried to some extent. As a result, separator layersthat were closely attached to the both surfaces of the positive andnegative electrodes were formed. The positive and negative electrodeshaving the separator layers attached to both surfaces thereof werelaminated; thereafter, the thickness of the separator layer between thepositive and negative electrodes was measured to be about 140 μm.

A battery was produced in the same manner as Example 1, except that thestack of the positive and negative electrodes with the separatorsattached to both surfaces thereof was spirally wound to form anelectrode group. This battery was denoted as “Battery E”.

Battery E was evaluated in the same manner as Example 1.

The discharge capacity of Battery E at the initial charge/dischargecycle was about 1000 mAh.

The average cycle number needed to reach a discharge capacity of 500 mAhwas 350.

INDUSTRIAL APPLICABILITY

According to the present invention, a low-cost nickel-metal hydridestorage battery having a long cycle life and small self-discharge can beobtained.

1. A nickel-metal hydride storage battery comprising a positiveelectrode containing nickel hydroxide, a negative electrode containing ahydrogen storage alloy, and a separator containing a gel electrolyteinterposed between said positive and negative electrodes, characterizedin that said gel electrolyte comprises a water absorbent polymer, awater repellent material and an aqueous alkaline solution.
 2. Thenickel-metal hydride storage battery in accordance with claim 1, whereinsaid water absorbent polymer comprises a cross-linked polymer having atleast one kind of monomer unit selected from the group consisting of anacrylic acid salt unit and a methacrylic acid salt unit.
 3. Thenickel-metal hydride storage battery in accordance with claim 1, whereinsaid water repellent material comprises at least one selected from thegroup consisting of fluorinated carbon and fluorocarbon resin.
 4. Thenickel-metal hydride storage battery in accordance with claim 1, whereinsaid separator comprises a core material comprising a non-woven fabric,and said non-woven fabric comprises polyolefin or polyamide.
 5. Thenickel-metal hydride storage battery in accordance with claim 1, whereinsaid separator is attached to a surface of at least one of said positiveand negative electrodes.
 6. The nickel-metal hydride storage battery inaccordance with claim 1, wherein said separator further comprises atleast one binder selected from the group consisting of polyethylene,polypropylene, carboxymethyl cellulose, styrene butadiene rubber andpolyvinyl alcohol.
 7. The nickel-metal hydride storage battery inaccordance with claim 1, wherein said separator has an air permeabilityof 5 to 100 ml/cm²·s under a pressure difference of 120 to 130 Pa. 8.The nickel-metal hydride storage battery in accordance with claim 1,wherein said separator has a thickness of 20 to 200 μm.
 9. A method forproducing a nickel-metal hydride storage battery comprising: (1) a firststep of producing a positive electrode containing nickel hydroxide and anegative electrode containing a hydrogen storage alloy; (2) a secondstep of producing a separator containing a gel electrolyte from amixture obtained by mixing a water absorbent polymer, a water repellentmaterial and an aqueous alkaline solution; (3) a third step of obtainingan electrode group by laminating said positive and negative electrodeswith said separator interposed therebetween; and (4) a fourth step ofassembling a nickel-metal hydride storage battery by using saidelectrode group.
 10. The method for producing a nickel-metal hydridestorage battery in accordance with claim 9, wherein said second stepcomprises a step of forming a sheet-like separator from said mixture.11. The method for producing a nickel-metal hydride storage battery inaccordance with claim 9, wherein said second step comprises a step ofapplying said mixture on a surface of at least one of said positive andnegative electrodes, and thereby forming a separator attached to saidsurface.